The present invention is directed to a method of detecting an object by employing electromagnetic radiation. More particularly, the invention concerns using radio waves for identifying a depth and material of an object buried in the earth, although application of the invention is not limited thereto.
Ground penetration radar technology has the potential to be of great use, for example, in geographical exploration to locate deposits of minerals, such as coal.
Two fundamental problems which have plagued ground penetration radar technology are penetration and focus. Electromagnetic waves of low frequency can penetrate the ground, but the use of low frequencies degrades the ability of the system to resolve small objects. Further, when the electro-magnetic waves do penetrate, the returned signal reverberates (reflection jitter) and is out of focus. Media dispersion coupled with antenna and target dispersion can additionally degrade radar performance.
One of the major problems in object detection is that the antenna interferes with a true measurement, as in the old galvanometer problem, wherein when an unknown resistor is to be measured, part of the reading is due to the meter's internal resistance. The same is true for all ground penetrating radar, only it is more complex. Obtaining a true measurement is hindered for reasons including: the fact that the height of the antenna above the ground affects impedance, the antenna looks like a target because of self reflection, and the dispersion within the antenna distorts all the variables.
Prior attempts to improve resolution have been to increase bandwidth by using impulse radars. However, this has only compounded the problem of dispersion correction. Another trick was to increase the bandwidth in the time domain by using a resistive antenna. This type of antenna has more bandwidth with less dispersion, but operates at a great cost of efficiency, and, moreover, does not solve the ground interaction problem.
An illustration of the ground radar penetrating problem is shown in FIGS. 1(a) and 1(b). The source 10 is accordance with space, phase, frequency, electric currents and magnetic currents. The unknown medium 12 is defined in accordance with permittivity, permeability, space, phase and frequency. Source 10 has definable currents (a) which generate unique fields (b) in the sub surface 16. These fields induce both electric and magnetic currents (c) in the targets 18, which are any dielectric anomaly in the ground. These targets generate fields (d) in space that the source can receive. The process from (a) to (c) is unique assuming a single source 10. But since the number of targets, i.e., sources below surface 14, is unknown, the process from (c) to (d) is not unique unless all possibilities of space, phase and frequency are sampled.
Such problems and some related solutions are discussed in the following articles, which are herein incorporated by reference: J. Durnin, "Exact Solutions for Nondiffracting Beams, The Scaler Theory," J. Opt. Soc. Am. A, vol. 4, no. 4, pp. 651-661, (April 1987); J. Durnin et al., "Diffraction Free Beams," Physical Review Letters, vol. 48, p. 1499, (April 1987); D. A. Hill, "Electromagnetic Scattering by Buried Objects of Low Contrast," IEEE Trans. Geosci. Remote Sensing, vol. 26, pp. 195-203, (March 1988); and D. A. Hill, "Near-Field Detection of Buried Dielectric Objects," IEEE Trans. Geosci. Remote Sensing, vol. 26, pp. 364-368, (March 1988). The following book is also incorporated by reference: D. M. Kerns, Plane-Wave Scattering-Matrix Theory of Antennas and Antenna-Antenna Interactions, National Bureau of Standards, NBS Monograph 162, June 1981.
The complexity of electromagnetic waves has caused designers to oversimplify the systems. A radar system must have enough degrees of freedom to be able to solve the complex sensor problem. The method of the present invention uses the additional tool of spatial modulation. Synthetic aperture radars have long used spatial modulation to resolve multiple targets in a single real beam width. The present invention also uses the spatial domain to reject unwanted reflections from sources other than the target and to aid in canceling out antenna dispersion.